1. Field of the invention
The present invention relates to a magnetic recording/reproducing apparatus using a perpendicular magnetic recording double-layer film medium with a soft underlayer and a high-sensitive reproducing (read) head using a magneto-resistive effect element (hereinafter referred to as MR read head or MR reproducing head), and a magnetic recording/reproducing signal processing circuit for the magnetic recording/reproducing apparatus.
2. Description of the related art
Perpendicular magnetic recording is a promising technology for achieving a high-density magnetic recording system. Combining a single pole write head and a double-layered film structure medium comprised of a soft-under layer and a recording magnetic layer provides a practical approach for the perpendicular magnetic recording system. In this recording system, a recording (write) magnetic field generated from a main pole of the head forms a magnetic path in which the magnetic field is induced to the underlayer disposed on the rear of the recording magnetic layer and returned from an auxiliary pole to the recording head. By switching the direction of recording magnetic field, the recording magnetic layer is magnetized in two directions towards the thickness of the medium in correspondence with the recording information code, thereby storing information. In such recording by means of the recording head and medium structure, an intensive and steep perpendicular recording (write) magnetic field can be applied to the recording magnetic layer, so that high-resolution information storage can be achieved. Moreover, when magnetized recording information is reproduced from the perpendicular magnetic recording medium recording the information, as described above, by the high-sensitive MR reproducing head using the MR device, a reproduced signal from the head has a rectangular-shaped signal waveform corresponding to the magnetized recording pattern which is sensed immediately by the head.
In the case of a conventional longitudinal magnetic recording system, with use of a combination system of a partial-response equalizing system and a maximum-likelihood decoding system, SN quality of the reproduced signal is improved and such signal processing system is widely used for a high reliable data reproduction. On the other hand, the signal processing system adapted for data detection of the reproduced signal by a perpendicular magnetic recording system as described above is heretofore unknown well, but several techniques such as the partial response class 1 used in an optical recording/reproducing apparatus, an extended partial response channel (JP-A-11-66755) thereof and a technique similar to integral signal detection, have been already proposed because the reproduced signal contains a large amount of DC signal component and has the rectangular shape approximated by a low-pass fittered waveform of a recording magnetized pattern and the recording current waveform.
Further, there has been proposed a processing method similar to a conventional signal processing system, the processing method using a signal processing system similar to the longitudinal recording system to previously process the reproduced signal from the perpendicular recording medium with a differentiation given, as a purpose of processing the reproduced signal is simplicity, and generating a pulse-shape signal waveform similar to the reproduced signal from the longitudinal recording system in pseudo manner.
As described above, the reproduced signal in the perpendicular magnetic recording system has a signal containing a large amount of rectangular-shape DC signal component. However, a lot of disturbance factors such as various kinds of noise from the recording medium and signal distortion due to low-frequency signal component loss in a signal transmission channel such as a reproduced signal amplifier are localized in the vicinity of the DC component of the reproduced signal. To eliminate the influence of the disturbance factors, an equalization process is performed for obtaining a waveform having such a frequency characteristic that the DC signal component of the reproduced signal is cut off and the region near the low-frequency signal component is suppressed.
In the signal processing system such as the partial-response class 1, the extended partial response channel thereof or the integral signal detection, it is, however, impossible to remove the DC component. Moreover, in the signal processing system in which the reproduced signal is differentiated before the equalization, the influence of noise and distortion in the low-frequency region can be eliminated because the DC component is cut off, but there is still a problem that the data error rate increases because the high-frequency noise component is emphasized by the differentiating process.
An object of the present invention is to provide a partial-response and maximum likelihood signal processing system capable of achieving a high-reliable data detection upon a signal reproduced from a perpendicular magnetic recording head medium system using a double-layer film recording medium and a single pole recording head/MR reproducing head. In order to achieve the object, there is provided means capable of reducing a noise and distertion on the reproduced signal and reducing a data detection error rate at the maximum-likelihood decoding because a waveform equalization condition for better adaptation to the reproduced signal is applied to the reproduced signal waveform of the high-density perpendicular magnetic recording system and the noise characteristic thereof in a partial-response equalization system.
To eliminate the influence of noise and distortion localized in the low-frequency region, the present invention provides the partial-response equalization system and an equalization circuit, in which the DC component can be removed from the equalized signal while the effect of suppressing increase of noise in the high-frequency region is sustained.
The partial-response equalization circuit in the present invention is constituted by a so-called transversal type filter having delay circuit elements connected multistageously for delaying an input signal for one bit-interval, a plurality of multipliers for multiplying each input signal, which is delayed substantially for one bit-interval, by a predetermined tap coefficient {h1, h2, h3, . . . hL}, and an adder for adding up the input signals multiplied by the tap coefficients respectively. The present invention has a feature that the tap coefficients satisfy the relation h1+h2+h3 . . . hL=0. With the tap coefficients determined in the aforementioned manner, the DC component can be cut off from the equalized signal waveform.
Moreover, a noise inputted into the maximum-likelihood decoder is whitened more optimally, so that the error rate of detection data is reduced. In addition, with the signal processing performed by use of the equalization system according to the present invention, the reliability of data detection can be made higher and the reproduced signal with a lower signal-to-noise ratio can be allowed compared with the disclosed technique in the related art. Hence, a magnetic recording/reproducing apparatus for achieving higher-density information storage can be provided.